As mentioned in the introduction of this section, authentication involves the process of verifying the identity of the person or entity with whom you are communicating. Certificates are a common way to achieve authentication.
A certificate is a set of data that identifies an entity, contains that entity's public key (a cryptographic key), and is issued by a trusted organization or entity called a Certification Authority (CA) only after the CA has verified that the entity is who it says it is. The data set includes the entity's public cryptographic key. When the sender of a message signs the message with its private key (and sends a copy of its certificate with the message), the recipient of the message can use the sender's public key (retrieved from the certificate) to verify that the sender is who it says it is.
On a network, there is often a trusted application, managed by a CA and running on a secure computer, that is known as a certificate server. Such an application has access to the public key of each user. Certificate servers dispense messages known as certificates, each of which contains the public key of one of its client users. Each certificate is signed with the private key of the Certification Authority.
Additional information on authentication through the use of certificates is presented in Authentication Using Certificates.